At the heart of electronics lies the ability to control electron flow across a solid structure. Electron flow is constantly switched on and off in transistors within integrated circuits. The performance of such circuits is related to the number of transistors in a given volume. It is well known, however, that as their size approach atomic scale, conventional transistors cannot operate normally and will increasingly fail due to uncontrolled electron scattering within the transistor.
Graphene is a stable, flat, and inert two-dimensional material that offers room temperature ballistic transport well over one micron, longer than in any other known material. See A. S. Mayorov, R. V. Gorbachev, S. V. Morozov, L. Britnell, R. Jalil, L. A. Ponomarenko, P. Blake, K. S. Novoselov, K. Watanabe, and T. Taniguchi, “Micrometer-scale ballistic transport in encapsulated graphene at room temperature,” Nano Lett., Vol. 11, pp. 2396-2399, 2011; and A. M. Song, “Room-temperature ballistic nanodevices,” Encyclopedia of Nanoscience and Nanotechnology, Vol. 9, pp. 371-389, 2004. Such properties make graphene a great candidate material for ballistic transistors, in which electrons can travel between source and drain contacts with little, if any, uncontrolled scattering.
The main obstacle preventing a breakthrough in graphene-based nanoelectronics is that graphene is a semi-metal without any transport gap needed to be able to shut off current. Engineering a gap in graphene without degrading its otherwise exceptional transport properties has proven to be a hard task that has yet to be solved.
To create such a gap, a lot of effort has been devoted to making graphene nanoribbons that are structurally isolated.
One approach to doing so has been through lithographic cutting. See C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, and W. A. de Heer, “Electronic confinement and coherence in patterned epitaxial graphene,” Science, Vol. 312, pp. 1191-1195, 2006; and M. Y. Han, B. Özyilmaz, Y. Zhang, and P. Kim, “Energy band-gap engineering of graphene nanoribbons,” Phys. Rev. Lett., Vol. 98, p. 206805, 2007.
Another approach has used nanoparticle cutting. See S. S. Datta, D. R. Strachan, S. M. Khamis, and A. T. C. Johnson, “Crystallographic etching of few-layer graphene,” Nano Lett., Vol. 8, no. 7, pp. 1912-1915, 2008; L. Ci, Z. Xu, L. Wang, W. Gao, F. Ding, K. F. Kelly, B. I. Yakobson, and P. M. Ajayan, “Controlled nanocutting of graphene,” Nano Res., Vol. 1, no. 2, pp. 116-122, 2008; and L. C. Campos, V. R. Manfrinato, J. D. Sanchez-Yamagishi, J. Kong, and P. Jarillo-Herrero, “Anisotropic etching and nanoribbon formation in single-layer graphene,” Nano Lett., Vol. 9, no. 7, pp. 2600-2604, 2009.
Other methods for making structurally isolated graphene nanoribbons involve unzipping of nanotubes, see D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii, J. R. Lomeda, A. Dimiev, B. K. Price, and J. M. Tour, “Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons,” Nature, Vol. 458, pp. 872-876, 2009; and L. Jiao, L. Zhang, X. Wang, G. Diankov, and H. Dai, “Narrow graphene nanoribbons from carbon nanotubes,” Nature, Vol. 458, pp. 877-880, 2009; or bottom-up synthesis, see J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. Feng, K. Mullen, and R. Fasel, “Atomically precise bottom-up fabrication of graphene nanoribbons,” Nature, Vol. 466, pp. 470-473, 2010; and M. Koch, F. Ample, C. Joachim, and L. Grill, “Voltage-dependent conductance of a single graphene nanoribbon,” Nat. Nanotech., 2012. DOI: 10.1038/nnano.2012.169).
Still other methods for making structurally isolated graphene nanoribbons use chemical methods within functionalized graphene. See R. Ruoff, “Calling all chemists,” Nat. Nanotech., Vol. 3, pp. 10-11, 2008; A. K. Singh and B. I. Yakobson, “Electronics and magnetism of patterned graphene nanoroads,” Nano Lett., Vol. 9, pp. 1540-1543, 2009; and W.-K. Lee, J. T. Robinson, D. Gunlycke, R. R. Stine, C. R. Tamanaha, W. P. King, and P. E. Sheehan, “Chemically isolated graphene nanoribbons reversibly formed in fluorographene using polymer nanowire masks,” Nano Lett., Vol. 11, pp. 5461-5464, 2011.
Irrespective of the method in which they are made, the edges in all these nanoribbons are hard boundaries that restrict transport to one dimension, and unless the edges are smooth on atomic scale, edge roughness will significantly degrade the transport properties. See D. Gunlycke, D. A. Areshkin, and C. T. White, “Semiconducting graphene nanostrips with edge disorder,” Appl. Phys. Lett., Vol. 90, p. 142104, 2007.